Individual study: Over-sowing with native Indian blanket Gaillardia pulchella results in suppression of invasive bastard cabbage Rapistrum rugosum along a roadside near Austin, Texas, USA
Simmons M.T. (2005) Bullying the bullies: The selective control of an exotic, invasive annual (Rapistrum rugosum) by oversowing with a competitive native species (Gaillardia pulchella). Restoration Ecology, 13, 609-615
Control of invasive plants by mechanical, biological or herbicide treatments can be expensive and sometimes detrimental to non-target species. Evidence suggests that over-sowing infested areas with native species with characteristics similar to those of the invasive may reduce productivity of the invasive whilst avoiding environmental damage. Bastard cabbage Rapistrum rugosum (an annual Eurasian species), is invasive in some areas of the USA. Herbicide control is problematic due to high mortality of native species and perpetuation of disturbed conditions that facilitates bastard cabbage regeneration. Field experiments were therefore undertaken to see if sowing native Indian blanket Gaillardia pulchella over established seedling colonies of R.rugosum had any controlling effect.
Study site: Experiments were conducted along a 300 m long rural roadside embankment near Austin, Texas. Rainfall during the study period (October 2000-May 2001) at 616 mm was higher than the 30-year average for the same months (476 mm).
Seed-sowing: In October 2000, three densities (0, 2, and 10 g/m²; n = 30) of Indian blanket seed were hand-sown in 0.25 m² plots randomly placed along a 300 m transect running along the embankment. The transect ran through am area of a more or less homogeneous, recently germinated R.rugosum seedlings. The following late spring (May 2001), plots were harvested, samples dried and weighed, and stems (individuals) for each species counted.
Along with R.rugosum, three other species (all natives) were co-dominate in the experimental plots: G.pulchella itself, horsemint Monarda citriodora and bluebonnet Lupinus texensis.
Sowing native Indian blanket over established R.rugosum seedlings resulted in significant reductions of R.rugosum. At the highest G.pulchella sowing rate (10 g/m²) there was a 72% reduction in R.rugosum shoot mass equivalent to an estimated 83% decrease in seed set. There was no significant suppression of native species within the plots.
At this site, as well as reducing bastard cabbage, the advantages of sowing with Indian blanket over conventional control (such as herbicide application) was that it avoided damage to non-target plants and avoided perpetuation of a disturbed (early-successional) state that might encourage recruitment of bastard cabbage thus aggravating the problem.
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